4. 2. (e+e) by using

Initial State Radiation (e+e

(‘radiative return’ to )

Conventionally (eehadrons, Q2hadr) is determined by an energy scan

at DANE not foreseen for the near future,

DANE has been designed for high luminosity at the  resonance

But, alternative approach (‘radiative return’)

Run at fixed energy and exploit the process

e+ehadrons + with the  emitted in the initial state(ISR)

to reduce c.m. energies of the colliding e+ e

andvaries between

H(Q2, cos o) radiation function

EVA MC Generator

NLO calculations Kühn et al. 2001

O(2) ISR radiative corrections Jegerlehner et al.2001

ISR and energy scan

The method using ISR is not a surrogate for an energy scan, but is complementary to it

It does not make obsolete an energy scan neither to study the use of  data (±→X±↔ee→ Xo)

ISR has the merits that

the error of the luminosity enters the Q2 spectrum only once, it is the same for the full c. m. energy range of electron positron annihilations

the error of the electron (positron) energies enters the Q2 spectrum only once

the data are taken as a by-product within the standard research program of KLOE without changing any part of the experimental set-up

ISR has the constraints

to be restricted to Q2 (  resonance)

todepend on the precise knowledge of the photon emission in the inital state including radiative corrections to higher orders

EVA MC generator, Kühn et al. 1999, 2001

Jegerlehner et al. 2001

decays and e+ e annihilation

MC studies with the EVA generator have been performed to demonstrate the feasibility to determine

(e+e)

in the process e+e with less than 1%

to reject FSR and other backgrounds below 1%

to achievestatistical and systematic errors below 1%

Also the possibility to measure the luminosity with better than 1% has been studied with MC calculations and with real data

It turned out that the required accuracy can be achieved by selecting appropriate phase space regions

4.3. Experimental results

total integrated luminosity in 1999: L ~ 2.4 pb-1

total integrated luminosity in 2000: L ~ 25 pb-1

total integrated luminosity in 2001: L ~ 200 pb-1

first  analysis has been performed with

17 pb-1of reconstructed data (Nov.- Dec. 2000)

detected in DC(with high momentum resolution),

’s detected in EmC

(at large angles with low energy resolution)

’s not detected in EmC (corresponding to small angles)

Particle separation (electrons, muons and pions)

likelihood method

neural net

kinematical fit

•Besides ISR (enhanced by the  resonances)

two more physical processes contribute to the final state:

•

Final State Radiation FSR

(implemented in the EVA MC code,

checked by looking at the pion pair asymmetryin the polar angle distribution, due to ISR-FSR interference)

•Direct decay → f→ 

(implemented in the EVA MC code by analysing the channel → f→ 

The relative contributions of the three processes depend strongly on the

photon polar angle and on the value of the two pion invariant mass:

•ISR contribution is peaked at small photon polar angles

•FSR and direct decaycontributions are mainly observed in the high

M region (M20.8 GeV2) (low energy photons) and for larger photon angles



4.3.2. Background reduction


4.3.3.ISR versus FSR (EVA - MC)and pion polar angle asymmetry

The model of FSR in MC is tested by looking

at the charge asymmetry of the pion pairs


4.3.4. Differential cross sectionsee



4.3.6. Pion form factor |F(Q2)|2

has been compared with
J. H. Kühn and A. Santamaria, Z. Phys. C48 (1990) 445

m=0.773 GeV= 0.145 GeV

m=0.782 GeV= 8.5 10-3 GeV

m=1.37 GeV = 0.51 GeV

1.85 10-3 =  0.145

BWi = Breit-Wigner formulae


4.4.Summary and conclusions

hadr important for 2 problems of precision particle physics

to determine the hadronic contribution of (g 2): = 1.5 · 10 10

so far based exclusively on the data of Novosibirsk

expected error of Brookhaven exp. E821 aµ = 4 · 10 10

to determine the hadronic contribution of em()

constraining the Higgs mass, Weinberg angle etc.

At DANEKLOEmeasurements of hadronic cross sections(e+e)have been started

Phase I : hadr for E < 1 GeV via Initial State Radiation

data takingsince 1999

Phase II : systematic energy scan ( 2 m < E < 1.4 GeV)

data taking not before 2004


Initial State Radiatione+e hadrons+allowsto scan theregion Q2 < 1 GeV while running at fixed energy

complementary to beam energy scan

MC studiesand first data takenwith of(e+e)using the processe+eindicate that KLOE is able to do this measurement with less than 1 % error

 analysis has been performed with 17 pb1of reconstructed data

total integrated luminosity in 1999: L ~ 2.4 pb1

total integrated luminosity in 2000: L ~ 25 pb1

good for 2 % error of (e+e-)

total integrated luminosity in 2001: L ~ 200 pb1

efficiencies, systematics, background under control

efficiencies eff: already at a few %, independent of MC

luminosity L: precision at the %level (< 2 %)

background dNbkg: small background from Bhabhas,

systematics syst: errors from , Q2, 

have been studied with MC

How to proceed further?

Final goal

to determine  (e+ e)at the level of0.3…0.5 %

in order to determine with better than 0.5 %

to reduce the error of the running Sommerfeld constant

Closest collaboration between theoreticians and experimentalists indispensable to achieve an accuracy of the hadronic corrections of better than 0.5 %

KLOE collaborates with the theoretical groups of

S. Jadach, CERN and Cracow

F. Jegerlehner, DESY Zeuthen

J. H. Kühn, Karlsruhe

Theoretical workon higher order radiative corrections

on MC generators

on QCD inspired models of FSR

Experimental work on precision data

on implementation of MC generators